This invention relates to a bidirectional bipolar transistor switch arrangement for use in customer end telecommunications equipment connected to a telephone line whose line feed polarity can be either positive or negative. In such communications equipment it may be necessary to provide a line powered switch to switch low level AC signals to an ancillary circuit element, when the telephone line current is present.
For such a purpose it is known to use contacts of an electromagnetic relay whose coil is incorporated in the telephone line loop. Such an arrangement is described in our co-pending Australian Pat. Appl. No. 36813/99, in which contacts of a line current sensing relay cause a capacitor to be inserted into a filter when an associated telephone is brought into an off-hook mode, to alter the characteristics of the filter.
There are, however, several disadvantages associated with relays, including reed relays, when used for this purpose. A major disadvantage is that such relays are barely sensitive enough for reliable operation at low current values, typically 20 to 50 mA. This is because the relay coil is serially connected in the line loop and its coil resistance therefore needs to be relatively low. Consequently the number of turns in the coil is small. Another disadvantage is that of contact unreliability. Further, the fast rise time of current when the contacts of the relay close may cause errors in some forms of signal transmission such as ADSL transmission.
It is also known to use JFET transistors for switching AC signals. While the drain/source of a JFET transistor is capable of conducting current in both directions, a control voltage must be applied to the transistor""s gate of a polarity that matches the JFET type (P-channel, N-channel). Generating a unipolar control voltage when the line feed polarity can be either positive or negative is difficult. Although two JFETs could be used, the relatively high turn-on voltage of JFETs make them less attractive for this application.
Another known AC switch is an optically coupled MOS switch, sometimes known as a MOS optocoupler. While this type of switch can also handle low level AC signals, its operating LED is typically unipolar which requires a rectifier bridge if the device is to operate with a line-feed polarity of either polarity. Such a rectifier bridge is disadvantageous in that it introduces a relatively high voltage drop. Further, this type of switch is relatively expensive.
It is therefore an object of the present invention to provide a low cost bidirectional line powered bipolar transistor switch arrangement that can reliably operate at low current values, and can operate with a line-feed polarity of either polarity without the need for a polarity guard.
According to the invention there is provided a bidirectional bipolar transistor switch arrangement comprising a current sensing resistance means of predetermined value connected between a first node and a second node, a first bipolar transistor whose base element is connected to said first node and whose emitter element is connected to said second node, and a second bipolar transistor whose base element is connected to said second node and whose emitter element is connected to said first node, the first and second bipolar transistor""s respective collector elements being connected together at a third node, said switch arrangement""s connection means being formed by said second node and said third node, whereby when direct current is caused to flow between said first node and said second node through said current sensing resistance means to produce a voltage across the first and second nodes of a magnitude such that, depending on its polarity, either said first transistor or said second transistor is turned on thereby in a forward and reverse conduction mode, thereby providing an AC conducting path between said connection means via the turned on transistor""s emitter/collector junction.